In recent years, for lightening weight for improving the fuel efficiency of automobiles etc., Al alloys and other light metals or high strength steel sheet have been increasingly used for auto parts and members.
However, Al alloys and other light metals have the advantage of being high in relative strength, but are remarkably higher in price compared with steel, so their use has been limited to specialty applications. To promote reduction of the weight of automobiles in a broader area, use of inexpensive high strength steel sheet is being strongly sought.
In general, materials become worse in formability the higher the strength. Ferrous metal materials are no exception. Attempts have been made to achieve both high strength and high ductility up until now. Further, another characteristic sought in a material used for auto parts is, in addition to ductility, burring. However, burring also exhibits a tendency to fall along with higher strength, so the improvement of burring is also becoming a topic in use of high strength steel sheet fir auto parts. On the other hand, auto parts are comprised of press formed and other worked members assembled together by spot, arc, plasma, laser, and other welding. Further, recently, steel sheet has been welded together, then press formed in some cases. Whatever the case, the weld strength at the time of forming or the time of use assembled as a part is extremely important from the viewpoints of the forming limits and safety. Therefore, in application of high strength steel sheet to auto parts etc., the burring and the weld zone strength also become important issues for study.
For high strength steel sheet excellent in burring, an invention adding Ti and Nb to reduce the second phase and cause precipitation strengthening by TiC and NbC in the main phase of polygonal ferrite so as to obtain high strength rolled steel sheet excellent in stretch flange formability has been proposed (Japanese Unexamined Patent Publication (Kokai) No. 6-200351).
Further, an invention adding Ti and Nb so as to reduce the second phase, make the microstructure acicular ferrite, and cause precipitation strengthening by TiC and NbC to obtain high strength, hot rolled steel sheet excellent in stretch flange formability has also been proposed (Japanese Unexamined Patent Publication (Kokai) No. 7-11382).
On the other hand, as technology for improving the weld zone strength, an invention complexly adding Nb and Mo so as to suppress the softening of the weld zone in steel sheet has been proposed (Japanese Unexamined Patent Publication (Kokai) No. 2000-87175).
Further, an invention making active use of the precipitation of NbN to suppress softening of the weld zone so as to obtain steel sheet comprised of ferrite and martensite has also been proposed (Japanese Unexamined Patent Publication (Kokai) No. 2000-178654).
However, in suspension arms, front side members, and steel sheet for other parts, burring and other formability and the strength of the weld zone are very important. In the above prior art, the two characteristics could never simultaneously be satisfied. Further, for example, even if the two characteristics are satisfied, provision of a method of production enabling production inexpensively and safely is important. The above prior art must be said to be insufficient.
That is, in the invention described in Japanese Unexamined Patent Publication (Kokai) No. 6-200351, to obtain a high stretch flange formability, an area ratio of at least 85% of polygonal ferrite is essential, but to obtain a 85% or higher polygonal ferrite, the steel has to be held for a long time to promote the growth of the ferrite grains after hot rolling. This is not preferable in operating costs.
Further, in the invention described in Japanese Unexamined Patent Publication (Kokai) No. 7-11382, due to the microstructure with the high dislocation density and the precipitation of fine TiC and/or NbC, just a ductility of about 17% at 80 kgf/mm2 is obtained and the formability is insufficient.
Further, these inventions do not allude at all to softening of the weld zone. On the other hand, the invention described in Japanese Unexamined Patent Publication (Kokai) No. 2000-87175 does not describe anything regarding the improvement of burring.
Further, the invention described in Japanese Unexamined Patent Publication (Kokai) No. 2000-178654 relates to a complex ferrite-martensite structure steel, which is clearly different from the technology of the present invention for obtaining a microstructure of steel sheet excellent in burring.